This invention relates to a knock sensor system and, in particular, to a knock sensor system including circuitry for detecting and responding to a knock sensor disconnect condition.
FIG. 1 shows a system designed to sense the premature detonation ("knock") of an engine and means for generating signals to correct for the knock. A knock sensor 10, which may generally be a microphone (or a piezoelectric device, as shown in FIG. 2) and which is located on or near an automobile engine, produces a knock signal (ek) in response to the "knocking" of the engine. The knock sensor 10 is connected via line 101, which may be relatively long, to an electronic system 3 mounted on a printed circuit board (PCB). The electronic system 3 comprises an integrated circuit (IC) 5, also referred to as a chip, and a microprocessor 29. As described below, the chip 5 contains circuitry for processing and amplifying signals produced by knock sensor 10 and coupled to the chip via line 101. The signals (ek) derived from the sensor 10 are applied to an amplifier 12 whose output is applied to the input of an anti-aliasing filter 13 whose output is applied to the input of a programmable gain stage 14. In response to the knock signal ek the programmable gain stage 14 produces at its output an in-phase signal (ek1) and an out-of-phase signal (ek1b). The signal ek1 is applied to a bandpass filter 16a and the signal ek1b is applied to a bandpass filter 16b. The output of filter 16a produces a signal identified as V.sub.IN and the output of filter 16b produces a signal identified as V.sub.INB. Theoretically, in-phase signal V.sub.IN should be the exact complement (or inverse) of the out-of-phase signal V.sub.INB. Input signals V.sub.IN and V.sub.INB are then applied to a rectifier section 18 which controls the application of the signals V.sub.IN and V.sub.INB to an integrator 20. Integrator 20 includes a positive signal integrator 20a and a negative signal integrator 20b. The rectifying circuit 18 includes circuitry for comparing V.sub.IN and V.sub.INB and switches for enabling the positive going portion of signals V.sub.IN and V.sub.INB to be applied to integrator 20a and the negative going portion of signals V.sub.IN and V.sub.INB to be applied to integrator 20b. As a result, the outputs of integrators 20a and 20b function to increase the positive and negative amplitude of the knock signal over selected integrating intervals.
The outputs of integrators 20a and 20b are fed to a differential to single-ended amplifier 22 whose output charges a storage capacitor C24. The voltage developed across capacitor 24 is applied to and drives a buffer 26 having an output 27 which is fed to microprocessor 29 which further processes the output voltage from the capacitor 24 to control (reduce) the engine knock.
A problem exists when the knock sensor is disconnected from the circuit. When a disconnect condition exists, noise signals at the input of the amplifier 12 and other signals picked up by the system may produce false "knock" signals which may be fed to the microprocessor 29 and disable the engine at an inopportune time. It is therefore desirable to sense a knock sensor disconnect condition and in response thereto prevent the knock sensor electronic system from supplying potentially dangerous signals.
It is therefore an object of this invention to sense a knock sensor disconnect condition and, in accordance thereto, prevent the electronic system from interfering with the operation of the engine. However, generating and distributing a signal to prevent the electronic system from interfering with the engine operation, normally requires that at least one special line be run. Where the electronic system responsive to the knock signals is built on a chip mounted on a printed circuit board, all the pins of the chip/package may be dedicated for specific uses. It is therefore undesirable and/or problematic to produce a separate output requiring a pin/terminal of the chip and/or package.